ABSTRACT: Abundance, biomass, population structure, growth, mortality and reproduction of 4 fiddler crabs (Ucacumulanta, U.maracoani, U.rapax and U.vocator) were studied in the Caeté mangrove estuary, North Brazil, to understand their life history strategies and species distribution in the mangrove forest. Three zones were sampled in the mangrove ecosystem: high intertidal forest (F), small creeks running through the forest (SC), and open mud banks of large tidal creeks (LC). Significant differences between zones were found in silt/clay, water and organic content of the sediment. Species zonation was pronounced: U.rapax and U.vocator were restricted to the high intertidal forest (differing preferences for organic and water content of the sediment probably allows these 2 species to coexist); U.cumulanta and U.maracoani were mostly restricted to the mid and low intertidal (the sunny mud banks of large creeks were completely dominated by the latter, probably due to its greater tolerance to high temperatures). Fiddler crab biomass was highest in LC, followed by F and SC (72.9, 19.5, and 10 g m2, respectively), while abundance was highest in LC, followed by SC and F (79.1, 69.2, and 59.3 ind. m2, respectively). Population structure of all species was strongly skewed towards the smallest size classes, and sex ratios were 1:1 in U.maracoani and U.vocator, with significantly more females in U.cumulanta and U.rapax. All fiddler crabs studied were fast-growing, and had short life spans and high mortality rates. Von Bertalanffy growth parameters, growth performance, and total mortality rates were calculated for all species. L∞, K, and Munros φ ranged from 11.1 mm carapace width (CW), 4.24 and 1.7 yr1 in U.cumulanta females up to 35.2 mm CW, 2.03 and 3.2 yr1 in U.maracoani males. Total mortality rates (Z) varied between 4.9 and 10.1 yr1, with females having generally 15 to 20% higher mortality rates than males, except for U.cumulanta. All species except U.vocator reproduced mainly during the dry season, but breeding was always prolonged and sometimes year-round, e.g. in U.rapax, 1 of the 2 species occurring in the forest. The second forest species, U.vocator, had a clear spawning peak towards the end of the rainy season and temporal separation of reproduction may also promote coexistence of the 2 species.